The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display having a reduced brightness distribution inconsistency in a surface light source, and an improved display characteristic.
A liquid crystal display has such features as being thin, light-weighted and power-saving, and therefore is used widely as a display for a wrist watch, calculator and so on. Especially, an active driving display driven by thin-film transistors (TFT) is replacing a CRT which has been a major display for a word processor, personal computer and so on.
The liquid crystal display can be configured in various display modes such as twisted nematic (TN) mode, vertical orientation mode, and in-plane switching (IPS) mode. Recently, the vertical orientation mode and the in-plane switching mode attract attention for their wideness of view angle.
The liquid crystal display comprises a liquid crystal display element which, controlled by electric signals for example, passes or blocks light, and a surface light source for example.
As is written in page 117 to page 123 of the "Liquid Crystal Parts and Materials Business Frontier", a publication by the Industrial Research Association, the surface light source has two types, namely direct type and guide board type. In the guide board type, a board type surface light source has an edge mounted lamp, and the light is provided through the guide board. With this arrangement, a portion closer to the lamp has a higher brightness, and so in actual application, a mask for example is used in order to reduce (uniformalize) brightness distribution.
In the direct type, brightness is high in right above the lamp, so the mask is provided over the lamp to directly shade the light from the lamp, thereby providing brightness distribution control. Further, other improvements are incorporated in these surface light sources, including provision of an optical film such as a light-diffusing sheet, and optimization in optical design.
However, use of the light diffusing sheet and the like adds manufacturing cost, and therefore the number of sheets for controlling the brightness distribution is often decreased. Currently, there is still approximately 10 to 30% of inconsistency in an in-plane brightness distribution.
FIG. 12 shows a construction of the liquid crystal display. In this figure, a member indicated by numeral 1 is a surface light source (sectional view), a member indicated by numeral 2 is outgoing light from the surface light source, a member indicated by numeral 3 is a liquid crystal display element (liquid crystal cell, sectional view) passing or blocking light depending on electric voltage, and a member indicated by numeral 4 is outgoing light from the liquid crystal display. FIG. 13(a) is a relative brightness distribution in the liquid crystal display from its left end (X) to right end (Y) along a horizontal scanning line, FIG. 13(b) shows a relative transmittance distribution of the liquid crystal display, and FIG. 13(c) shows a relative brightness distribution of the surface light source.
The brightness of the liquid crystal display is expressed as "surface light source illuminance.times.transmittance of the liquid crystal display element". Therefore, if the surface light source having the brightness distribution as shown in FIG. 13(c) is used with the liquid crystal display element having a uniform relative transmittance distribution as shown in FIG. 13(b), the brightness distribution of the surface light source is directly reflected in the display screen, resulting in such problems that display is not made uniformly, gradation is not displayed as designed, and good display characteristics are not achieved. Although the use of the light diffusing sheets for example reduces inconsistency in the in-plane brightness distribution of the surface light source, a large number of the sheets are necessary if the brightness distribution is to be totally uniformalized, resulting in high manufacturing cost. For these reasons, it has been difficult to provide a liquid crystal display having a small in-plane brightness distribution inconsistency manufactured at a low cost, only by improving the in-plane brightness distribution of the surface light source.
An object of the present invention is to offset the brightness distribution inconsistency of the surface light source by controlling the in-plane transmittance distribution of the liquid crystal display element, thereby obtain a liquid crystal display manufactured at a low cost and high in display performance in which the in-plane brightness distribution is improved to be superior to the surface light source brightness distribution.